Resonant inverters advantageously have low switching losses and low switching stresses However, resonant operation is complex due to the fast dynamics of the high-frequency resonant tank circuit; and, hence, control is difficult. Disadvantageously, when input power or output load conditions vary, output voltage or current control cannot be achieved through the use of usual control techniques. For example, one known resonant inverter output load voltage or current control method is to vary the frequency of the rectangular wave signal supplied to the resonant circuit by the inverter via closed loop control. Commonly assigned U.S. Pat. No. 4,541,041, issued on Sept. 10, 1985 to J. N. Park and R. L. Steigerwald, which is hereby incorporated by reference, discloses in part such a frequency control technique. Briefly explained, the resonant nature of the circuit allows for control of output voltage or current through variation of the frequency at which the inverter's controllable switch means operate Such a frequency control method has been found satisfactory under normal output load conditions for particular types of resonant inverters (i.e., heavy or medium load conditions for a series resonant inverter and light load conditions for a parallel resonant inverter). The drawback to frequency control, however, is that it may be inadequate to maintain a desired output voltage or current under extended output load conditions (i.e., light or no load conditions for a series resonant inverter and heavy load conditions for a parallel resonant inverter).
In particular, frequency control of a series resonant inverter will normally be adequate to maintain a desired output voltage during heavy or medium load conditions (i.e., low load resistance); that is, for heavy or medium load conditions, a series resonant circuit has a high quality factor Q and thus a good dynamic range of voltage or current change as frequency is varied. However, under extended or light output load conditions (i.e., high load resistance) the series resonant circuit exhibits a low quality factor Q and thus only a small dynamic range of output voltage or current change can be achieved as a function of frequency. As a result, for a series resonant inverter, it may be impossible to maintain a desired output voltage or current under light load and no load conditions solely with frequency control.
A resonant inverter control which provides an improved dynamic range of output voltage or current control is disclosed in U.S. Pat. No. 4,672,528, issued June 9, 1987 to J. N. Park and R. L. Steigerwald and assigned to the assignee of the present invention. This patent, which is hereby incorporated by reference, describes a resonant inverter which is controlled using either a frequency control mode or a phase shift control mode. In the frequency control mode, output voltage is controlled by varying the frequency of the rectangular wave signal supplied to the resonant circuit within an operable range of the controllable switch means. Selecting means allows the control to operate in the phase shift control mode when the frequency of the rectangular wave signal is at an extremity of the operable range of the controllable switch means.
Another method of resonant inverter control, which is derived from optimal control theory and state plane analysis, is presented in "Resonant Power Processors: Part II-Methods of Control" by Ramesh Oruganti and Fred C. Lee, 1984 Industry Applications Society Proceedings, pages 868-878, and is hereby incorporated by reference. According to this method, hereinafter designated "optimal trajectory control" to be described in detail below, each state trajectory corresponds to particular values of instantaneous resonant tank energy, output voltage, output current and switching frequency. These state trajectories are utilized to define a control law for the inverter control system which enables a series resonant inverter to respond quickly to load and control requirements. Disadvantageously, however, in the method of "optimal trajectory control", as it presently exists, the controlled range of output voltages is limited in the same manner as the hereinabove described conventional frequency control method.
Accordingly, it is an object of the present invention to provide a new and improved resonant inverter exhibiting an improved dynamic range of output load voltage control.
Another object of this invention is to provide a new and improved resonant inverter control which utilizes a combination of optimal control methods and phase modulation to maintain output load voltage substantially constant during all loading conditions.
Still another object of this invention is to provide a new and improved resonant inverter control which switches automatically between different control means to maintain a substantially constant output load voltage.
Yet another object of the present invention is to provide an improved method of controlling a resonant inverter in order to maintain a desired output load voltage.